Bucket wheel excavators, as typified by conventional wheel ditching machines and strip mine excavating machines where the bucket wheel is carried on a boom, have historically employed a passive, gravity discharge method. This method drops the discharging material toward the center of the wheel as each bucket reaches the top of its rotation by uncovering an opening in the inward wall, or back, of the bucket. The material then falls onto a conveyor at the interior of the wheel and is carried away. The nature of this discharging action, wherein gravity overcomes centrifugal force, favors relatively large diameter digging wheels which turn at lower rotational speeds. Any desired cutting speed can be achieved at a relatively low rotational speed by increasing wheel diameter, thereby reducing the influence of centrifugal force on the material being discharged.
Recent years have seen the advent of another form of bucket wheel excavator as disclosed in Satterwhite U.S. Pat. No. 3,896,571, wherein closely spaced bucket wheel sections are closely and rigidly coupled to the main frame of the machine. This later type discharges material outwardly, well after the bucket passes the top of its rotation In these machines, the bucket assemblies are again an integral part of the wheel structure, with their volume defined by the wheel sides and three surfaces referred to as the back, bottom, and lip. The lip includes a cutting edge, and is set at a shallow angle to the periphery of the wheel, providing an entry ramp for material into the bucket. The bucket bottom is fixed in position and the bucket back pivots on an axis located behind the lip of the leading adjacent bucket. The bucket back is moved by a pushrod which works on a cam roller near the wheel center so as to push material from the bucket in a positive manner at the appropriate angle of wheel rotation.
The bucket back displaces material outwardly so that, on the downward travel of the bucket, gravity causes the material to slide down and to the outside. Conveyors are located just outside the arc of the wheel, at a height that is necessarily below that of the wheel center, so as to catch the discharge and carry it off. In practice the optimum height of this discharge point has been found to be at an elevation of approximately one-third of the diameter of the digging wheel. It is necessary, however, to maintain a sufficient digging wheel size to allow physical room for a conveyor system to fit under its discharge without ground interference, and this larger diameter tends to reduce the centrifugal forces related to discharge. In any case, the influence of centrifugal force assists, or at least does not oppose, the discharge of material in this type bucket wheel excavator, as it did with the earlier type machines. In practice, the belt conveyor length needed to carry material to an elevated rear transfer point determines the overall machine length in that, if material is received at a lower point, the machine becomes consequently longer. The bucket discharge elevation issue has been addressed more recently by an articulated bucket design which raises discharge to the height of the wheel center at the expense of greater complexity.
The drive for rotating the bucket wheel sections is accomplished by means of reduction gearing which is internally mounted within the confines of the wheel assemblies and is thus difficult to inspect and maintain. This forward concentration of mass also tends to make the machine front-heavy, so that a counterbalancing rear overhang is needed to distribute undercarriage loading.
A significant aspect of these excavators is a capability for excavating harder formations than is possible with the boom mounted bucket wheels. This capability is attributable to the chatter-free presentation of cutting edges to the work face and to the higher cutting forces which can be applied with a close-coupled machine configuration.
An object of the present invention is therefore to achieve the potential of close-coupled, rigidly supported cutting edges while avoiding the penalties of unnecessary size, weight and complexity. This calls for excavating means with an elevated discharge height and elimination of the complexity of articulated buckets. A related object is to provide a more accessible drive for this excavating means and moderate the forward weight bias by moving this drive rearward.
The present invention accomplishes these objects with an excavating machine having closely spaced digging chain assemblies rigidly supported by the main frame at the front of the machine. These chain assemblies include large capacity, multiple pitch, transverse buckets which are adapted to accommodate chain wear and to conform to varying chordal dimensions as the digging chains pass through straight runs and around the upper driving sprockets and lower digging sprockets, which may be toothed or not. The digging sprockets are of sufficient diameter to provide rigid support to the chains along the depth of the work face. The driving sprockets are positioned above and behind the digging sprockets so that material is discharged outwardly from the chain path by both gravitational and centrifugal forces as the buckets pass over the driving sprockets.
The driving sprockets are preferably smaller in diameter than the digging sprockets so as to limit the overall height of the machine and enhance the centrifugal discharge effect. In effect, this relatively smaller diameter also provides a drive line reduction stage. Power input to the driving sprockets may be accomplished by various hydraulic, electrical or mechanical means, but in any case, the mass thereof is moved significantly toward the rear of the machine.
A moldboard blade trims material left between the bucket chain assemblies so as to smooth the cut surface and a skid plate immediately behind the moldboard blade bears against the cut surface to react and support the cutting loads of the bucket chain.
In an alternate embodiment, particularly suited to bucket excavators configured for an elevated bucket discharge, a bucket wheel assembly is carried on frame extensions between the closely spaced sections, as in prior practice. In this embodiment these spaces also accomodate digging sprockets which rotate the bucket wheel assembly. As with the bucket chain embodiment, the chains driving these sprockets take the form of digging chains which break up the material standing in their path so that this material falls into the adjacent buckets. Thus, the width of the space between bucket wheel sections is not critical to excavator performance.
The grade cut by the subject invention in all embodiments can be defined and controlled in the manner taught by Bryan U.S. Pat. No. 5,058,294, the contents of which are hereby incorporated in this application by reference.